Exhaust gas recirculation (EGR) is used in many internal combustion (IC) engines, and particularly gasoline and diesel engines. In an EGR system, a portion of an engine's exhaust gas is recirculated back to the engine cylinders. Therefore, at a time when a cylinder allows fuel, oxygen and other combustion products into the combustion chamber for ignition, vehicle exhaust is also allowed to enter the chamber.
The introduction of vehicle exhaust into the combustion chamber has a number of consequences. One consequence is that the introduced exhaust displaces the amount of combustible matter in the chamber. Because the exhaust gases have already combusted, the recirculated gases do not burn again when introduced to the chamber. This results in a chemical slowing and cooling of the combustion process by several hundred degrees Fahrenheit. Thus, combustion of material in the cylinder results in a same pressure being exerted against the cylinder piston as results from combustion without the recycled exhaust, but at a lower temperature. The lower temperature leads to a reduced formation rate for nitrous oxide emissions. Thus, the EGR technique results in less pollutants being emitted in an engine's exhaust.
Additionally, the introduction of recirculated exhaust gas into an engine cylinder allows for an increase in engine performance and fuel economy. As the combustion chamber temperature is reduced, the potential for harmful “engine knock” or engine detonation is also reduced. Engine detonation occurs when the fuel and air mixture in a cylinder ignite prematurely due to high pressure and heat. In engine detonation, instead of an associated spark plug controlling when a cylinder's fuel is ignited, the ignition occurs spontaneously, often causing damage to the cylinder. However, when the combustion chamber temperature is reduced due to EGR, the potential for engine detonation is also reduced. This allows vehicle manufacturers to program more aggressive (and hence, more efficient) timing routines into an associated spark timing program. Because of the aggressive timing routines, the vehicle's power control module (PCM) has a greater advance notice and thus more time to take measures to prevent engine detonation. The aggressive timing routines can also result in higher cylinder pressures leading to increased torque and power output for the vehicle. For these and additional reasons, high levels of EGR are especially useful when applied to turbocharged or supercharged engines.
Accelerator pedal “tip-out” is the well known phrase referring to the action of a driver releasing the pedal from a depressed position to a zero (i.e., completely released) or near zero (i.e., mostly released) position. Upon a pedal tip-out, the driver expects the engine's output power to be abruptly reduced. It is a well-known technical challenge to manage EGR flow for the pedal tip-out situation.
When the engine operates at a partial load, it is desirable to have a high EGR rate for better fuel economy and lower emissions. While at idle, however, the engine has little tolerance for EGR flow. When a pedal tip-out occurs, air already mixed with a high portion of recirculated exhaust gas in the intake manifold has to go through engine combustion to exit the vehicle. As such, there may be a delay before the throttle is completely closed to ensure that the recirculated exhaust gas exits the vehicle. Delaying throttle closing to keep the engine running at the partial load, however, may result in safety concerns. On the other hand, an immediate throttle closing will cause engine combustion instability.
Due to this dilemma, a common approach in today's vehicles is to limit the EGR rate to a containable level even though a higher EGR rate will be more beneficial under most driving circumstance. Accordingly, there is a need and desire for an improved EGR scheme that is suitable for the pedal tip-out transition while also being optimized for normal situations (i.e., non-pedal tip-out situations).